Human embryonic stem (hES) cells are derived from the inner cell mass of pre-implanted blastocysts. They have been shown to differentiate into a variety of cell types that represent endoderm, ectoderm, and mesoderm origins via three-dimensional structures called embryoid bodies (EBs), which at least partially mimic the spatial organization of the embryo. Various lineages have been derived from hES cells, including neurons, cardiomyocytes, hematopoietic cells, osteogenic cells, hepatocytes, insulin-producing cells, keratinocytes, and endothelial cells. Furthermore, these cells appear to be weakly immunogenic, with absent MHC-II and only low levels of MHC-I molecules. Given their unlimited self-renewal and pluripotency capacity, hES cells represent a new and exciting avenue for stem cell therapy. In cell culture, hES cells can differentiate into endothelial cells through successive maturation steps. Therefore, the isolation and use of hES-derived endothelial cells (hESC-ECs) have potential therapeutic applications, including cell transplantation for repair of ischemic tissues and tissue-engineered vascular grafts.
Stem cell therapy is an exciting area of research that promises future treatment of many diseases. However, to fully understand the beneficial effects of stem cell therapy, investigators must be able to track the biology and physiology of transplanted cells in living subjects over time. At present, most cell therapy protocols require histological analysis to determine viable engraftment of the transplanted cells. The development of sensitive, noninvasive technologies to monitor this fundamental engraftment parameter will greatly aid clinical implementation of cell therapy.